Here’s a question that has bugged me for ages. Maybe Frank or someone may know the answer?

I understand one of the main scientific objectives of the famous Venus Transit voyages in the 1760s was to obtain observations from widely separated points on earth so that solar parallax could then help calculate the actual distance sun to earth and from that the size of the solar system?

All suggested earlier by Halley who couldn’t live long enough to see the results.

Any reason why they could not have used the more common Mercury transits to do this?

I’ve always assumed Mercury was too close to the sun, but is there a more fundamental reason?